This invention relates generally to the field of dental cutting tools and to a method for minimizing pain associated with the repair of dental caries. More specifically the invention relates to a dental bur that, independently of operator control, removes primarily carious dentin having a preselected hardness without removing normal dentin or significant amounts of pre-carious dentin, or any dentin having a hardness above the preselected hardness value.
Dental caries involve infectious microbiological disease wherein the infection results in localized dissolution and destruction of calcified tooth structures. When the lesion extends beyond the enamel or cementum to the underlying dentin, various changes occur in the dentin. In general, these changes are depth dependent. For example, bacterial infection tends to be higher in the superficial layers of dentin, i.e. those closest to the surface of the tooth (the dentin in those layers being hereinafter referred to as xe2x80x98carious dentinxe2x80x99), whereas deeper layers of dentin have lower levels of infection or may not be infected at all (the dentin in those layers being hereinafter referred to as xe2x80x98pre-carious dentinxe2x80x99). In addition, the superficial layers of carious dentin are greatly softened, and the hardness of the dentin gradually increases with the depth of the dentin until a level is reached where the dentin is of normal hardness. Thus, between the extremes of carious dentin (highest level of infection and lowest hardness) and that of normal dentin (no infection and highest hardness) is pre-carious dentin (low or no infection and intermediate hardness).
In most instances, it is medically advisable to remove substantially all carious dentin to ensure that the lesion does not expand, and to provide a good mechanical foundation for the restoration material. Removal of pre-carious dentin is not medically necessary in most instances because it is minimally or not at all infected (and thus does not pose significant danger of the lesion expanding), and also because its hardness is sufficient to provide a good mechanical foundation for the restoration material. Since carious dentin is not sensitive but pre-carious dentin is sensitive, a procedure that removes some or all carious dentin but does not remove most pre-carious dentin can be substantially pain-free while medically sound.
There are many common dental instruments used for removing carious dentin from teeth. Typically, however, high speed burs or other excavation tools are used to excavate the carious dentin. Unless the operator senses when a harder material is encountered and immediately ceases drilling, prior art burs will continue to cut through both carious dentin and pre-carious dentin into normal dentin during and after removal of the carious dentin. Because of the difficulty in sensing precisely when harder material is encountered, the side effect of using such a bur is cutting of pre-carious dentin and some cutting of normal dentin. In addition, this produces an infected and clogged bur that is relatively expensive to discard and difficult to sterilize.
U.S. Pat. No. 6,106,291 describes and claims a bur which is designed to deflect, deform or abrade upon encountering dentin having a preselected hardness corresponding to the lower limit of hardness for non-carious (i.e., normal) dentin. Thus, the bur is designed to cut through both carious and pre-carious dentin, but to deflect or abrade when it reaches normal dentin, and to do so independently of operator control. Such a bur avoids inadvertent and unnecessary removal of normal dentin, but does not distinguish between the carious dentin which must be removed to provide a sound base for placing a restoration and the pre-carious dentin which need not be removed during restoration. Nor does such a bur distinguish between various levels of hardness of carious dentin in those clinical procedures requiring only partial removal of carious dentin, for example, indirect pulp cap procedures.
The present invention provides an improved dental cutting tool which deflects, deforms or abrades when encountering material having a preselected hardness below the lower limit of hardness of normal dentin. Thus, the bur of this invention is capable of selectively differentiating between softer carious dentin having various levels of hardness and between carious dentin and harder pre-carious dentin, substantially irrespective of operator control. Further, by selecting a bur having a preselected hardness such that it deforms, deflects or abrades upon encountering material having a hardness corresponding to or lower than the lower limit of hardness of pre-carious dentin, thereby minimizing or eliminating the removal of pre-carious dentin, a restoration can frequently be accomplished painlessly or with minimal dental discomfort and without need for anesthesia.
The dental cutting tool (or xe2x80x9cburxe2x80x9d) of the present invention has a working surface that includes cutting elements adapted to deflect, deform, or abrade when encountering a material of hardness above a preselected value, wherein the preselected value is less than the lower limit of hardness of normal dentin. As a corollary, the preselected hardness may also be referred to as the bur""s xe2x80x98cutting hardness,xe2x80x99 reflecting the fact that the bur will cut or drill selectively only material of less than the preselected hardness. Since a bur is selected having a cutting hardness which is less than the lower limit of hardness of normal dentin, the bur will excavate all carious and/or pre-carious dentin having a hardness below the preselected value. When dentin of hardness above the bur""s cutting hardness is encountered, the cutting elements deflect, deform, or abrade, thus preventing damage to or removal of the harder dentin. More particularly, the present invention provides a dental bur with a cutting hardness in the range of about 1 to about 40 Knoop Hardness Number (KHN), whereas the lower limit of hardness for normal dentin is characteristically at or above about 50 KHN or 60 KHN. In a preferred embodiment, the bur""s cutting hardness is preselected at or slightly below the lower limit of hardness for pre-carious dentin, thus permitting painless repair of a carious lesion while removing most carious dentin. In a further embodiment, the bur""s cutting hardness is preselected at a value below the lower limit of hardness of pre-carious dentin, thereby permitting only partial removal of carious dentin for procedures such as indirect pulp cap. Thus, by selecting a bur of appropriate cutting hardness below the hardness of normal dentin, one may remove all or a portion of carious dentin and, optionally, a portion but not all of pre-carious dentin, thereby preventing removal of or damage to normal dentin.